The present invention relates to an image reading apparatus which reads an original mounted on a platen with a line-shaped photoelectric conversion element that moves along the platen, and more particularly, to an image reading apparatus provided with a transmissive light source unit which applies light from a light source unit to a transparent or translucent transmissive original mounted on a platen and reads an amount of transmission light with a photoelectric conversion element.
In general, an image reading apparatus, which scans and electrically reads an original mounted on a transparent platen such as glass with a line sensor such as a CCD and C-MOS sensor, has widely been applied to a facsimile, scanner and the like. A light source that applies light to an original on the platen is integrally attached to a carriage provided with optical elements to form an image on the line sensor, and an image of the original is read while the carriage is moved.
However, such an apparatus cannot accurately read a transparent (translucent) original such as a photograph and OHP because light from the light source passes through the original, and therefore, an apparatus has recently been known in which a light source unit is provided above a platen that is different from a side (under the platen) where a sensor is arranged, and light passed through the original from above to under is read with a photoelectric sensor. It is known that such an apparatus that reads a translucent film such as film has a structure where a light source unit is mounted on a carriage on one side of a platen while a reading optical unit is mounted on another carriage on the other side, the carriages are arranged to allow them to reciprocate in the subscanning direction along the platen, and an image is read while both the carriages are moved at the same speed in synchronization with each other.
However, it has recently been required to switch emission amounts of the light source corresponding to types of originals so as to read different types of originals, for example, negative film and positive film, with high precision in the same apparatus. For switching a plurality of provided source lamps, a mechanism has conventionally been known that switches light-source lamps with different wavelengths such as an R (Red) lamp, G (Green) lamp and B (Blue) lamp for an original with a color image, and proposed, for example, in Patent Document 1 (JP2003-158611). The Document 1 discloses a mechanism where a plurality of light-source lamps is provided to be movable inside a light-source carriage that moves along the platen, and the light-source lamps are switched by moving a position of a lamp holder provided with the plurality of light-source lamps, irrespective of a moving (scanning) mechanism of the light-source carriage.
[Patent Document 1] JP2003-158611 (FIG. 3)
When one is selected to switch from a plurality of light-source lamps corresponding to the type of original in applying light to a translucent original such as film and reading the light passed through the original with a photoelectric sensor, applying the mechanism as described in Patent Document 1 causes following problems: In the mechanism of the Document, a light-source carriage moving along the platen is provided with driving means for supporting a plurality of light-source lamps to be movable and moving the light-source lamps. Accordingly, the apparatus needs to be provided with mechanisms for supporting respectively a carriage (reading carriage) mounted with a photoelectric sensor, a carriage (light-source carriage) mounted with a light source unit, and a lamp holder mounted with lamps inside the light-source carriage, and with respective driving mechanisms.
Therefore, in addition to increased complexity and size in the apparatus, there are risks that distortion and/or color shading may develop on a read image when displacement occurs on three moving mechanisms due to rattling and impact. Further, since an emission amount of the light-source lamp has a parabolic characteristic in the subscanning direction of the line sensor, when misalignment occurs in switching the lamps or the characteristic of the emission amount shifts due to exhaustion of the lamp during the use on each of convertible light-source lamps, it is not possible to compensate for such misalignment or shift, and there is a fear that the read image may deteriorate with time.
Then, the present invention aims to switch light-source lamps by controlling movements of a carriage mounted with the light-source lamps in reading a translucent original (transmissive original) by moving the light-source carriage and the reading carriage so as to enable switching of the light-source lamps corresponding to types of originals without using a specific lamp switching mechanism. Accordingly, it is an object of the present invention to provide an original reading apparatus capable of reading an image with an amount of light corresponding to the type of original with a simplified structure. It is another object of the invention to provide an image reading apparatus, such as an image reading apparatus of a computer or the like, which is capable of executing at high speed a series of processing of selection of original type, light source setting, and image reading.